Musculoskeletal disorders affecting the bones and joints are a growing health problem. Osteoporosis affects over 10 million people in the United States. In addition, bone fractures result in over 3 million emergency department visits a year. Since our ability to treat and prevent these diseases is still very rudimentary, elucidating the mechanisms that regulate bone formation is crucial for understanding the pathologic changes in bone diseases and for developing targeted treatments to increase bone formation. The overall objective of this proposal is to define the biological processes regulating the formation of musculoskeletal tissues in a newly developed model of G-protein signaling in bone growth. Prior results showed that expression in osteoblasts of an engineered receptor activated solely by synthetic ligand (RASSL), "Rs1," could dramatically increases bone mass in Rs1 transgenic mice. This proposal will define the basis of this phenotype in three specific aims: 1) determine the cellular mechanisms of Rs1-induced bone formation with a combination of mouse and in vitro tissue culture models;2) determine how Gs signals affect osteoblast differentiation in mouse and embryonic stem cell models;and 3) identify endogenous GPCRs mimicked by Rs1 using directed expression analysis of Rs1-expressing osteoblasts. Successful completion of these studies will identify new interactions between G-protein signaling pathways and other mechanisms of bone growth, while identifying native GPCRs that may be important clinical targets for increasing bone growth. In addition, the results will improve our understanding of how osteoblasts, adipocytes, and bone marrow interact in normal bone. The proposed research is part of a coordinated career development plan to prepare the candidate to be an outstanding, independent physician-scientist through research, coursework, and tutorials. The candidate will acquire additional training under his mentors'guidance on the utilization of fundamental and translational methods for studying developmental diseases. Finally, a mentoring committee will ensure that the candidate acquires the skills and experience necessary to successfully direct an independent research program by the conclusion of the Career Development Award.